To be honest, I don't know a good font from a bad one, but one thing I do know is that one font is not enough… In your CSS that is. I am using Linux here; it's very likely that I don't have Arial or Times New Roman, or any of the other fonts the Microsoft world takes for granted. So when your CSS calls for a single Microsoft font, I'm likely to get the standard Firefox selection, which is probably a very bad serif font. The guys on Apple Macs will be little better off, and they are more likely to scoff. If you want to be as helpful as possible, you should use standard font families and provide a good list of fall-back fonts for your readers. Wiser minds than mine have put together the following lists.

The standard CSS knows about five font families, and it's probably best to stick to those; Too many fonts on a page are one of the most obvious signs of bad design. The five families are: serif, sans-serif, monospace, cursive and fantasy. You could just specify one of those, and the browser would pick a font to use. Theoretically they are all available, practically cursive and fantasy are new additions and most browsers wont have them. It isn't really a good idea anyway, because better fonts exist, and your readers may have them installed. The best thing to do is to provide a list of fonts in your order of preference. Because you will want to use those five generic typefaces as fall-backs, you are limited to those five categories of font families.

The reader's browser starts at the left-hand end of the list and uses the first font it finds that it has available. So the fonts on the left are the best choice, and the ones on the right are for making the best of a bad job. Notice that the first three lines use the default browser font as the fallback. However not all browsers have default fonts for cursive and fantasy, so both of those fall back ultimately on sans-serif.

For what it is worth, this is how those font-family lines look on your browser. Of course they may look different on someone-else's.

The serif font looks like this.

The sans-serif font looks like this.

The monospace font looks like this.

The cursive font looks like this.

The fantasy font looks like this.

These font samples below are mostly from Wikipedia, under the Wikimedia Commons or public domain license. Click them for a better look.

Serif

The list is: Times New Roman, Palatino, Book Antiqua, Times, roman, serif

Times New Roman is a Microsoft font. Times is also a Microsoft font but pre-dates Microsoft's use of TTF so it will appear blocky in some sizes. Roman is the free version. Book Antiqua is similar to Palatino, but Wikipedia doesn't have a handy sampler. It's the middle one of the three. Click for a better look.

Times New Roman

Palatino

Book Antiqua

Sans-Serif

"Myriad Web" is a paid-for typeface. It is not likely that your readers have it, but it is certain that if they do then they will appreciate your using it. Arial Unicode is a little different from Arial, but I couldn't find an example. It's not likely to be obvious anyway. I think Trebuchet and Trebuchet MS are the same font.

Myriad Web

Lucida Sans

Trebuchet MS

Verdana

Arial

Helvetica

Geneva

Monospace

Courier New is a Microsoft font, Courier is the pre-TTF blocky version. Monotype is the Microsoft version of Andale Mono. Console is the typeface used for the Blue Screen of Death. Note that the font "monospace" is quoted to distinguish it from the similarly named keyword at the end of the list.

Fantasy

You are not going to get anything like a consistent look with this list; the fonts are all very different from one another, from the gothic fraktur and blackletter to the ultra-modern Gadget (which I believe you'll find on Apple kit). So if you are looking for a certain look and feel, you'll need to research your own font list. But if all you want is a bit of spot interest, this list is probably good enough.